The sixth mass extinction is in full swing: all over the world, species are dying out faster than the normal species change of the past million years. A new study now illustrates just how dramatic it looks in the land vertebrates. According to this, more than 540 species of land vertebrates became extinct in the 20th century, and of 515 other species fewer than 1,000 still exist today. These species have lost more than 237,000 of their populations since 1900. If the last populations of these acutely endangered vertebrates are also lost, this could also result in a whole cascade of further species losses in their habitats, warn the scientists.
In the course of the earth’s history, nature has repeatedly experienced large extinctions – triggered by asteroid strikes such as at the end of the Cretaceous period, volcanic eruptions or climate changes. But when man stepped on the stage of evolution, he came into a world that was more diverse than ever before. “Today, only around two percent of all species that have ever existed on earth live, but the absolute number of species is now greater than ever,” explains Gerardo Ceballos from the Autonomous National University of Mexico and his colleagues. However, that is changing rapidly because people are increasingly interfering in the living spaces and livelihoods of their animal and plant contemporaries. “The extinction rate today is one hundred to a thousand times higher than the background rate of species extinction in the past more than ten million years,” say the researchers. “Every time a species or population disappears, it also erodes to some extent Earth’s ability to maintain ecosystem functions because each species is unique.”
515 land vertebrate species are on the brink of extinction
To find out how fast the pace of species extinction is progressing today, Ceballos and his team used the example of land vertebrates to investigate how many species have died out since 1900 and how many are on the brink of extinction. As such, they defined species of which there are fewer than 1,000 representatives worldwide. This corresponds to the number of individuals from which the International Union for Conservation of Nature (IUCN) classifies a species in its red list as “threatened with extinction”. “If the number of individuals in a population or species drops too far, it can become too small to still survive and reproduce,” the researchers explain. Because in such a small population, genetic diversity and resistance to disturbances are greatly reduced.
The evaluation of around 29,700 animal species showed that 543 land vertebrate species had already died out in the course of the 20th century. “In the normal course of evolution, it would have taken up to 10,000 years for so many species to become extinct,” say Ceballos and his colleagues. The researchers determined a similarly high number for the proportion of acutely threatened animal species: “5151 species only have less than 1000 remaining individuals – this corresponds to 1.7 percent of the land vertebrate species”, report the researchers. In more than half of these 515 species, the number of animals had even dropped to less than 250 specimens. Another 388 land vertebrate species have fewer than 5000 specimens.
Birds have the largest share of the acutely endangered species, followed by amphibians, mammals and reptiles. The geographic distribution of these animal species at the brink of extinction differs slightly: the majority of them are found in the tropical and subtropical regions of the world. However, the majority of birds are concentrated in South America and Oceania, while a particularly large number of mammals in Asia and Oceania are threatened, as the researchers report. Only one percent of these 515 species live in Europe.
(Video: Stanford University)
237,000 populations are gone
“The species on the verge of extinction have not only lost drastically to individuals, but also to populations,” said Ceballos and his colleagues. This was shown when they compared the historical and current distribution areas of 48 mammal species and 29 bird species among the 515 species. The result: “These species have lost 95 and 94 percent of their geographic distribution since 1900,” the researchers report. “Around 3600 populations of the examined mammal species and 2930 populations of bird species have disappeared.” If one extrapolates this for all 515 acutely threatened land vertebrate species, they could have lost a total of 237,000 populations. “And these losses do not only occur with obscure species that hardly interest anyone. Instead, there are many populations of large and well-known animals, ”emphasize the scientists. These include, for example, the Sumatran rhinoceros or the giant tortoises from Espaniola.
According to Ceballos and his team, these numbers are cause for concern. Because, as they explain, it is unlikely that these 515 acutely endangered species will remain. If these die out, other species living in their ecosystems and also already strongly decimated could also be dragged into the “abyss”. “Extinction leads to extinction,” is how the researchers characterize this cascade of disappearance. “So what we do over the next 20 years to slow the current mass extinction could determine the fate of millions of species,” says Ceballos. “It is still possible, but the time window is closing rapidly.” The researchers are suggesting measures to conclude a binding global agreement and, for example, to generally ban the wildlife trade. The protection of endangered species should also be given higher priority at the national level. “It is up to us to decide what kind of world we want to leave behind for future generations – a sustainable one or a desolate one,” says co-author Peter Raven of the Missouri Botanical Garden.
Source: Gerardo Ceballos (Universidad Nacional Autónoma de México, Mexico City) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.1922686117